Dr. Hare's Second Letter to Prof. Faraday. IGO 



jionderable, it must of necessity be imponderable. When 

 light is communicated from a luminous body in the centre of 

 an exliausted sphere, agreeably to the undulatory hypothesis, 

 its efficacy is dependent on the waves excited in an interve- 

 ning imponderable medium. Agreeably to your electro-polar 

 hypothesis, the inductive efficacy of an electrified body in an 

 exhausted sphere would be due to a derangement of electric 

 equilibrium, by which an opposite electric state would be pro- 

 duced at the surface of the containing sphere from that at the 

 centre (26, 27). This case you consider as one of extraordi- 

 nary induction; but when air is admitted into the hollow 

 sphere, or when concentric spheres are interposed, you hold 

 it to be a case of ordinary induction. Let us then, in the 

 case of the luminous body, imagine that concentric spheres of 

 glass are interposed, of which the surfaces are roughened by 

 grinding. In consequence of the roughness thus produced, 

 the rays, instead of proceeding in radii from the central ball, 

 would be so refracted as to cross each other. Of the two 

 instances of illumination, thus imagined, would the one be 

 desci'ibed as ordinary, the other as extraordinarij radiation'^ 

 But if these epithets are not to be applied to radiation, where- 

 fore, under analogous circumstances, are they applicable to 

 induction? Wherefore is induction, when acting through a 

 plenum, to be called ordinary, and yet, when acting through 

 a vacuum, to be called extraordinary? In the well-known 

 case of the refracting power of Iceland spar, light undergoes 

 an ordinary and extraordinary refraction ; not an ordinary 

 and extraordinary radiation. The candle, of which, when 

 viewed through the spar, (wo images are seen, does not radi- 

 ate ordinarily and extraordinarily. 



51. If there be occasionally, as you allege (21), large inter- 

 vals between the particles of radiant heat, how can the distances 

 between them resemble those existing between particles acting 

 at distances, which are not sensible? The repulsive reaction 

 between the particles of radiant caloric, as ilescribed by you 

 (21), resembles that which I have supposed to exist between 

 those of electricity ; but I cannot conceive of any description 

 less suitable for either, than that of particles which do not act 

 at sensible distances. 



.52. Aware that the materiality of heat, and the Newtonian 

 theory, which ascribes radiation to the projection of lieat or 

 light-producing particles, have been questioned, I should not 

 have appealed to a doctrine which assumes both the materi- 

 ality oi' heat and the truth of the Newtonian theory, had not 

 you led the way; but, agreeably to the doctrine and theory 

 alluded to, I cannot accord with you in perceiving any 



